The present invention relates to a glass composition for magnetic recording medium having excellent water resistance and weather resistance, and in particular, it relates to a glass substrate for magnetic recording medium obtained by subjecting a glass blank having surface flatness and small surface warpage obtained by the float process, to tempering.
In general, aluminum substrates are widely used as magnetic recording medium. Thin plate is desired for the recently used substrates, and high surface flatness is desired for increasing recording density. A glass substrate is suitable as such a magnetic disk substrate, and in particular, forming this glass substrate by the float process is an advantageous method from the standpoint of the quality and cost.
If a blank with great warpage is used, the blank must be ground to correct the warpage. Because a blank formed to have high flatness by the float process does not need such correction, it is possible to produce a glass substrate for magnetic disk using a thin blank, making it possible to decrease cost for grinding.
At present, such glass substrates for magnetic recording medium are used after subjecting a glass substrate having a soda-lime glass composition, which is molded by the float process, to chemical tempering.
That is, in the glass substrates for magnetic recording medium, tempered products are mainly used., that are produced by immersing glass substrates in a molten salt containing a monovalent metal ion having an ionic radius larger than that of the monovalent metal ion contained in the glass composition to thereby exchange the metal ions in the glass for the metal ions in the molten salt.
However, problems are pointed out that if such glass substrates are subjected to a tempering treatment, weathering resistance (water resistance) deteriorates in high temperature and high humidity environment.
Further, where a glass substrate is used, thermal expansion of the glass should match that of a metal part, such as stainless steel or aluminum, which constitutes a magnetic disk drive.
U.S. Pat. No. 4,156,755 describes lithium-containing ion-exchange-reinforced glass as a tempered glass having excellent weathering resistance. This U.S. patent discloses at column 7, lines 2 to 15 a glass composition containing, in % by weight, 59-63% SiO2, 10-13% Na2O, 4-5.5% Li2O, 15-23% Al2O3, and 2-5% ZrO2, Al2O3+ZrO2 is 19-25% by weight and Na2O/ZrO is 2.2-5.5.
For example, JP-A-62-187140 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a process for producing a tempered glass. This JP-A discloses on page 1, left column, lines 5 to 16 a process for producing a tempered glass containing, in % by weight, 64-70% SiO2, 14-20% Al2O3, 4-6% Li2O, 7-10% Na2O, 0-4% MgO and 0-1.5% ZrO2.
However, the glass compositions disclosed in the examples of U.S. Pat. No. 4,156,755 and JP-A-62-187140 described above require high temperature for melting and forming, and it is not easy to produce a thin sheet having less warpage by the float process.
JP-A-5-32431 describes (on page 2, left column, lines 2 to 7) another tempered glass, and discloses a tempered glass article containing, in % by weight, 62-75% SiO2, 4-12% Na2O, 4-10% Li2O, and 5-15% Al2O3 and 5.5-15% ZrO2, wherein the weight ratio of Na2O/ZrO2 is 0.5-2.0 and the weight ratio of Al2O3/ZrO2 is 0.4-2.5. Because the composition disclosed in the above JP-A contains a large amount of ZrO2, crystals of ZrO2 tend to precipitate in a furnace in the case of producing using a melting furnace, and there was a problem on quality. That is, when a blank in which crystals of ZrO2 were precipitated is ground and polished to produce a magnetic disk substrate, ZrO2 crystals that are hardly polished off remain on the surface as projections and cause a head crush.
JP-B-47-1312 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined published Japanese patent applicationxe2x80x9d) discloses lithium or sodium aluminosilicate glass as a tempered glass plate suitable as a wind-screen of, for example, automobiles or aircraft and a process for producing the same. There is the disclosure on page 3, right column, lines 29-34 that particularly preferred glass is that its inner part comprises, on the weight of oxide basis, 2-6% LiO2, 5-10% Na2O, 15-25% Al2O3 and 60-70% SiO2, with the sum of Li2O, Na2O, Al2O3 and SiO2 being at least 95% by weight of the composition. The Example fallen in the above composition range, described in 3 in Table 1 of column 3 of the above JP-B needs high temperature for melting and forming and it is difficult to produce a high quality glass by the float process.
British Patent 1,322,510 discloses METHOD OF STRENGTHENING A GLASS ARTICLE AND A STRENGTHENED GLASS ARTICLE. One object of the patent is to provide a composition that can produce a glass sheet by Froucault process, Pennvernon or Pittsburgh process, Colburn process (page 1, lines 61-75), and there is the description as conditions for a glass composition therefor that a strain point is 450-550xc2x0 C., a working temperature is 980-1,150xc2x0 C. and a liquidus temperature is 1,100xc2x0 C. or lower. The composition described the British patent had the problems that the working temperature is high, the liquidus temperature is higher than the working temperature so that the composition is not suitable for forming into a thin plate with little warpage by the float process, and weathering resistance of the resulting tempered glass is poor.
Further, in aiming at the improvement of recording density in a magnetic disk substrate, there is a tendency to further decrease a distance between a substrate and a magnetic head, and then the cleanness of the surface of a substrate has become an issue. If any foreign matter is adhered to the surface of a substrate, the head strike against the foreign in reading and writing, causing head crush. To avoid this, the demand for cleanness of the substrate surface has been increasing. Washing with acid is an effective for the improvement of the cleanness. H2SO4, HF, NHO3 or the like is used the acid, and HF and H2SO4 are particularly effective.
If glass having poor acid resistance is washed with acid, latent scratch generates. On the other hand, glass having acid resistance more than necessary requires a concentrated acid for washing, and this results in problems on cost, on working, on environment and in waste treatment. An acid concentration suitable for washing is about 0.002-0.3% HF and about 1-20% for H2SO4.
JP-A-62-187140 supra has the description at column 3, upper right column, lines 3-6 that xe2x80x9cBecause reinforced glass having an extremely deep compression stress layer can be obtained with ease, the resulting reinforced glass can be used as a disk substrate that is required to have high reliability in strengthxe2x80x9d. Evaluation on resistance against 0.5% H2SO4 is made as the example. However, such is not evaluation for latent scratch by washing, but is evaluation that intends weathering resistance. There is no mention of HF resistance.
Accordingly, an object of the present invention is to provide a glass substrate for magnetic recording medium, that does not substantially contain ZrO2 which becomes a problem in the production using a melting furnace, has good water resistance or weathering resistance after tempering treatment and comprises a composition that has a melting temperature and a working temperature suitable for forming into a thin plate with little warpage by the float process.
The present invention further has an object to provide a glass substrate for magnetic recording medium, which has a coefficient of thermal expansion that makes it possible to use the substrate in combination with a metal article. The present invention also has an object to provide a glass substrate for magnetic recording medium, which is difficult to cause latent scratch in washing with an acid, particularly in using H2SO4 or HF.
The present invention has been made based on the above-described problems and demands in the prior art.
The present invention provides a tempered glass substrate for magnetic recording medium, that has a composition of, in % by weight:
61-70% SiO2,
9-18% Al2O3,
2-4.5% Li2O,
6-13% Na2O,
0-5% K2O,
10-16% R2O
(with the proviso of R2O=Li2O+Na2O+K2O),
0-3.5% MgO,
1-7% CaO,
0-2% SrO,
0-2% BaO,
2-10% RO
(with the proviso of RO=MgO+CaO+SrO+BaO),
0-2% TiO2,
0-2% CeO2,
0-2% Fe2O3, and
0-1% MnO,
with the proviso of TiO2+CeO2+Fe2O3+MnO=0.01-3%, does not substantially contain ZrO2, and has water resistance such that the sum of glass components eluted in a purified water when it is maintained in purified water at 60xc2x0 C. for 120 hours is 1 xcexcg/cm2 or less.
The particularly preferred embodiments are as follows.
a) The glass substrate in which an average coefficient of linear thermal expansion in a temperature range of 50-350xc2x0 C. is 80xc3x9710xe2x88x927/K or more;
b) The tempered glass substrate for magnetic recording medium, that has a composition of, in % by weight:
62-69% SiO2,
9-13.5% Al2O3,
2-4.5% Li2O,
7.5-12.5% Na2O,
0-2% K2O,
10-15% R2O
(with the proviso of R2O=Li2O+Na2O+K2O),
0-3.5% MgO,
2.5-6% CaO,
0-2% SrO,
0-2% BaO,
3-9% RO
(with the proviso of RO=MgO+CaO+SrO+BaO),
0-2% TiO2,
0-2% CeO2,
0-2% Fe2O3, and
0-1% MnO,
with the proviso of TiO2+CeO2+Fe2O3+MnO=0.01-3%, does not substantially contain ZrO2, and has water resistance such that the sum of glass components eluted in a purified water when it is maintained in purified water at 60xc2x0 C. for 120 hours is 1 xcexcg/cm2 or less;
c) The glass substrate in which an average coefficient of linear thermal expansion in a temperature range of 50-350xc2x0 C. is 84 xc3x9710xe2x88x927/K or more;
d) The glass substrate for magnetic recording medium, in which a melting temperature (temperature having a viscosity of 10 poise) of the glass composition is 1,550xc2x0 C. or lower, a working temperature (temperature having a viscosity of 104 poise) is 1,100xc2x0 C. or lower, and a liquidus temperature is the working temperature or lower;
e) The glass substrate for magnetic recording medium, in which a melting temperature (temperature having a viscosity of 102 poise) of the glass composition is 1,540xc2x0 C. or lower, a working temperature (temperature having a viscosity of 104 poise) is 1,055xc2x0 C. or lower, and a liquidus temperature is the working temperature or lower; and
f) The tempered glass substrate for a magnetic recording medium, characterized in that the Li ion and/or the Na ion in the vicinity of the surface of the glass substrate. are substituted with a monovalent metal ion having ionic radius greater than that of the Li ion so that the glass substrate has a compression stress in the vicinity of the surface thereof.
The reasons for limiting the composition of the glass substrate for magnetic recording medium according to the present invention are explained below.
SiO2 is a main component for consisting a glass and is an essential structural component. If the proportion is less than 61%, water resistance after ion exchange deteriorates and also acid resistance deteriorates. On the other hand, if it exceeds 70%, the viscosity of molten glass is too high, so that it is difficult to melt and mold and also the coefficient of thermal expansion becomes too small. For this reason, the SiO2 range is preferably 61-70%, and more preferably 62-69%.
Al2O3 is an essential structural component for increasing the rate of ion exchange and improving water resistance after ion exchange. If the proportion is less than 9%, its effect is not sufficient. On the other hand, if it exceeds 18%, the viscosity of molten glass is too high, so that it is difficult to melt and mold and also the coefficient of thermal expansion becomes too small. Further, acid resistance deteriorates. For this reason, the Al2O3 range is preferably 9-18%. In order to further increase the acid resistance, 9-13.5% is preferable.
Li2O is an essential structural component for conducting ion exchange and is also a component to improve melting property. If its proportion is less than 2%, surface compression stress after ion exchange is not sufficiently obtained, and further, melting property is poor. On the other hand, if it exceeds 4.5%, the water resistance after ion exchange deteriorates, and also the liquidus temperature rises, making it difficult to mold. For this reason, the Li2O range is preferably 2-4.5%.
Na2O is a component to improve melting property. If its proportion is less than 6%, its effect is insufficient. On the other hand, if it exceeds 13%, the water resistance after ion exchange deteriorates. For this reason, the Na2O range is preferably 6-13%, more preferably 7.5-12.5%.
K20 is a component to improve melting property but is not an essential component because it decreases the surface compression stress after ion exchange. For this reason, the K2O range is preferably 5% or less, more preferably 2% or less.
If the total R2O of Li1O+Na2O+K2O is less than 10%, the viscosity of molten glass is too high, so that it is difficult to melt and form and also the coefficient of thermal expansion becomes too small. On the other hand,if it exceeds 16%, the water resistance after ion exchange deteriorates. For this reason, the total R2O range of Li1O+Na2O+K2O is preferably 10-16%, more preferably 10-15%.
MgO is a component for improving melting property. If it exceeds 3.5%, the liquidus temperature elevates, making it difficult to form. For this reason, MgO is preferably 3.5% or less, more preferably 0.5-3%.
CaO is a component for improving melting property and is also an essential component for controlling the rate of ion exchange. If its proportion is less than 1%, its effect is not sufficient. On the other hand, if it exceeds 7%, the liquidus temperature raises, making it difficult to form. For this reason, the CaO range is preferably 1-7%, more preferably 2.5-6%.
SrO and BaO are components for improving melting property and also components effective to lower the liquidus temperature. However, the glass density increases and those lead to an increase of material cost. Therefore, SrO and BaO each are preferably 2% or less, more preferably 1% or less.
If the total RO of MgO+CaO+SrO+BaO is less than 2%, the viscosity-of molten glass is too high, so that it is difficult to melt and form. If it exceeds 10%, the liquidus temperature raises, making it difficult to form. For this reason, the total RO of MgO+CaO+SrO+BaO is preferably 2-10%, more preferably 3-9%.
Fe2O3 is that Fe2+and Fe3+are in an equilibrium state in a glass melt. Those ions greatly influence light transmission, especially infrared light transmission, in the melt. If the total iron exceeds 2% in terms of Fe2O3, absorption in the infrared region is too large, and the temperature distribution during melting or forming cannot be controlled, leading to deterioration in quality. For this reason, the total iron is preferably 2% or less in terms of Fe2O3.
TiO2, CeO2 and MnO are effective components to change the equilibrium between Fe2+and Fe3+and also change light transmission of light by interaction. However, If those contain in excess, the blank quality deteriorates and also material cost increases. Therefore, the TiO2 range is preferably 3% or less, more preferably 2% or less, and the CeO2 range is preferably 2% or less, more preferably 1% or less. Further, the MnO range is preferably 1% or less.
The glass substrate for magnetic recording medium of the present invention can further contain, other than the above components, coloring materials such as NiO, Cr2O3 or CoO, and refining agents such as SO3, As2O3 or Sb2O3, in a range that the characteristic of the present invention is not impaired.
Of those, SO3 has its origin in a sulfate used as a refining agent. Where the sulfate is used as a refining agent, if the residual amount in the glass is less than 0.05%, a refining effect is not sufficient. On the other hand, even if the residual amount exceeds 0.5%, the refining effect is equivalent, and further SOx contained in a waste gas in melting glass increases, which is unfavorable to the environment. For this reason, SO3 remained in glass is preferably 0.05-0.5%.
As2O3 or Sb2O3 that is commonly used as a refining agent is preferably 1% or less in view of their toxicity, and the amount thereof is desirably not higher than the amount contaminated from impurities, i.e., 0.1% or less.
B2O3, ZnO, P2O5, PbO, etc., which are highly volatile, corrode bricks of a glass melting furnace, and the volatile components gather on the ceiling of the furnace and fall down on the glass together with the bricks to deteriorate the glass quality. Therefore, the amount of those oxides are preferably not higher than the amount contaminated from impurities, i.e., 0.1% or less.
ZrO2 crystallizes during melting in a furnace. Where a blank containing such crystals is ground and polished to produce a magnetic disk substrate, ZrO2 crystals that are difficult to polish form projections on the surface of the substrate, causing a head crush. Therefore, ZrO2 is not substantially contained, except for impurities and one melted from the furnace material. However, impurities and that melted from the furnace material may contain in an amount of about 0.5% or less.
A magnetic disk glass substrate can be prepared by subjecting the glass blank having the composition as described in the claims of the present invention to a disk processing, followed by rough polishing, tempering and precise polishing. In this case, matching of a coefficient of expansion with a metal-made fixture such as stainless steel or aluminum is necessary. In this case, an average coefficient of linear thermal expansion in a temperature range of 50-350xc2x0 C. is preferably 80xc3x9710xe2x88x927/K or more, and further 84xc3x9710xe2x88x927/K or more is desirable.
For melting high quality glass, viscosity of the glass is that a melting temperature, that is, a temperature having a viscosity of 102 poise, is preferably 1,550xc2x0 C. or lower, and more preferably 1,540xc2x0 C. or lower. In order to form into a sheet having high flatness, particularly to form by the float process, it is preferable that a working temperature, that is, a temperature having a viscosity of 104 poise, is 1,100xc2x0 C. or lower and a liquidus temperature is not higher than the working temperature. Further, in order to form a thin plate having further decreased warpage, it is desirable that a working temperature is 1,055xc2x0 C. or lower and a liquidus temperature is not higher than the working temperature.
The tempering treatment using the above glass blank is that the glass substrate for magnetic recording medium of the present invention is maintained with a metal holder, pre-heated to 200-300xc2x0 C., and then dipped in a molten salt of an ion-exchange tank. The treatment temperature is 350-460xc2x0 C., and the treatment time is 0.5-8 hours. After the ion exchange treatment, the glass is taken out of the molten salt, cooled, washed with hot water, washed with water, and then dried. The molten salt used is NaNO3 salt, KNO3 salt or their mixed salt.
The tempered glass substrate for magnetic recording medium of the present invention has a surface stress of at least 10 kg/mm2, so that it is difficult to form scratch on the substrate surface and also the substrate is difficult to break.
The glass substrate for magnetic recording medium requires storage stability for a long period of time, and water resistance (elution resistance of glass components from glass surface due to moisture) which is one evaluation of weathering resistance which withstands use under bad environment, i.e., high temperature and high humidity.
If the total amount of glass components eluted in water exceeds 1 xcexcg/cm2, precipitates of eluted alkali plus carbonate may be formed on the surface of the glass substrate for magnetic recording medium during storage. In a magnetic recording medium which is film-formed while retaining the precipitates, head of a magnetic disk contacts with the precipitates and is worn, shortening life of the head. Further, the head strikes against the precipitates, causing head crush. Furthermore, in a magnetic disk medium using a glass substrate for magnetic recording medium having a poor water resistance, water permeates between the glass substrate and a film from the edge portion, causing white fog which one kind of discoloration, and this may be a problem on product. Therefore, the total amount of glass components eluted is preferably 1 xcexcg/cm2 or less.
Where a glass substrate for magnetic recording medium having poor acid resistance is washed with an. acid, latent scratch are produced. If a magnetic film is formed on such a glass substrate for magnetic recording medium having latent scratch, the magnetic film is not completely adhered at the latent scratch-produced portion and such a disk cannot read and write, causing error. For this reason, the number of latent scratch on the surface of the glass substrate for magnetic recording medium is preferably 3/mm2 or less, more preferably 1/mm2, when visually observed with an optical microscope with a magnification of 200.